WASHINGTON (Reuters) - An experimental gel protected female monkeys from the AIDS virus in a test designed to mimic human sexual transmission, researchers said on Wednesday.

The gel uses an AIDS drug along with a zinc compound and protected all animals tested from infection with the monkey version of HIV, the researchers report in the Public Library of Science open-source journal PLoS ONE.

It "afforded full protection (21 of 21 animals) for up to 24 hours after two weeks of daily application," they wrote.

The gel uses a very small amount of active drug and thus might be safe and cheap, said the Population Council in New York, which led the study.

The study joins a growing body of experiments that are beginning to show progress in preventing AIDS, a fatal and incurable virus that infects 33 million people globally and which has killed 25 million.

Melissa Robbiani of the Population Council, who worked with the National Cancer Institute and other laboratories to test the gel, is hoping to test it on people.

In July researchers stunned AIDS experts when they found a similar gel using the Gilead Sciences AIDS drug tenofovir reduced HIV infections in women by 39 percent over two and a half years.

Non-profit groups are moving ahead to develop that gel and the U.S. Food and Drug Administration has given it fast-track designation.

"It is like a positive domino effect," said Bethany Young Holt, director of the Coalition Advancing Multipurpose Innovations, a women's health research and advocacy group and an expert on microbicides -- gels, creams or other products that protect against infection.

PROTECTING WOMEN

Most infections with the AIDS virus are in Africa and most new cases are among women infected during sex with men.

A microbicide could help protect against HIV while allowing a woman to get pregnant, and, if necessary, she could use the product without letting her partner know.

"Just the idea of having a product that a woman could use to address the issue of unplanned pregnancy and sexually transmitted disease as well, that would be an enormous benefit to women," Holt said in a telephone interview.

This product does not prevent pregnancy but the researchers are working on a combined gel or a vaginal ring that includes a contraceptive.

The drug used in the Population Council product, MIV-150, was developed by Swedish company Medivir , and is licensed to the Council.

Like other HIV drugs, it blocks the virus from reproducing. MIV-150 specifically stops infected cells from spewing out new viruses.

MIV-150 is not available as a pill, and the Population Council says this is an advantage because it will not reduce a woman's options for treatment later if she does become infected.

Tenofovir, the drug used in the one successful microbicide, is also available as a pill. The tenofovir microbicide also prevented herpes infections in half the women tested.

The second ingredient in the Population Council's microbicide, zinc acetate, is meant to prevent herpes although this was not tested in this particular study.

However, the combination of zinc acetate and MIV-150 worked much better than either ingredient used alone in preventing simian HIV in the monkeys.